The present invention relates in general to improving the performance of a loudspeaker system at lower frequencies, and more particularly concerns an improved loudspeaker system characterized by improved performance in the low frequency range that has structure which is relatively easy and inexpensive to fabricate.
A major problem in making a loudspeaker system for low frequency reproduction is obtaining a high output at low frequencies while limiting loudspeaker cone excursions. Typically, loudspeaker topologies are configured such that cone excursions are reasonably within the displacement limits of the attached motor structure such that sonic output is relatively free from audible distortion. The size of the displacement region must be sufficiently limited to keep the cost of manufacturing loudspeakers from becoming excessive.
Many prior art low frequency speaker systems comprise a simple woofer with no enclosure, for example in television and radio sets and some public address systems. A difficulty with these systems is that there is no means for preventing the radiation from the back of the speaker from canceling the radiation from the front. In such a system peak sonic output is limited by the requirement of very large cone excursions at low frequencies.
One prior art approach for reducing back radiation, and cone excursion, is to place the loudspeaker driver in a closed box, forming what is often called an acoustic suspension system. An acoustic suspension system provides a reactance against which the loudspeaker driver work, limiting the cone excursion and also preventing the radiation from the back of the loudspeaker from canceling that from the front.
Although this embodiment provides for increased low frequency output compared to the enclosureless embodiment, the low frequency peak output is still limited by the displacement region limits of the motor structure.
One prior art improvement on the acoustic suspension system is a ported enclosure system. A ported system typically includes a woofer in the enclosure and a port tube serving as a passive radiating means. The air in the port tube provides an acoustic mass that provides system designers with an extra reactance which can be used to tune the loudspeaker response, typically altering the frequency response at the low end. A ported system is characterized by a resonant frequency at which the mass of air in the port reacts with the volume of air in the cabinet to create a resonance (port resonance). At the port resonance the cone excursion of the loudspeaker is minimized. A ported system exhibits improved sensitivity at port resonance and decreased cone excursion. The result of the decreased cone excursion requirements at frequencies near the port resonance is an increase in low frequency peak output and a decrease in distortion when compared to the acoustic suspension systems. Another result of the improved sensitivity at port resonance is often an extension of the lower cutoff frequency of the loudspeaker to a lower value.
A dual-chamber system has also been used to improve the performance of an acoustic suspension system. Such systems are disclosed in U.S. Pat. No. 4,549,631, assigned to the same assignee as the present application, and incorporated by reference herein in its entirety. A dual-chamber system has an enclosure divided into first and second subchambers by a dividing member. The dividing member is formed with an opening which contains a loudspeaker, the loudspeaker being oriented such that one surface of the loudspeaker cone is exposed to the first subchamber, and the other surface of the loudspeaker cone is exposed to the second subchamber.
In some dual-chamber systems, the first and second ports directly couple the first and second subchambers to the region outside of the enclosure. In other systems, the larger subchamber is directly coupled to the region outside of the enclosure, and the smaller subchamber is coupled to the region outside of the enclosure via the larger subchamber.
In dual-chamber systems, subchambers are coupled to each other or to regions outside the enclosure either by ports or by equivalent drone cones. This results in further increases in low frequency sensitivity and peak output when compared to the simpler ported enclosure system.